1. Field of the Invention
The present invention relates to a laser diode driving circuit that drives laser diodes used as light sources for recording/reproducing or the like in optical communications, laser printers, CDs (Compact Disks), DVDs (Digital Versatile Disks) or the like.
2. Prior Art
When a laser diode is used as a light source for the recording/reproducing or the like in optical communications, laser printers, CDs, DVDs or the like, a pulse current flowing through the laser diode is as large as tens to hundreds of milliamperes, and is switched at high speed (specifically, at a rise/fall time of 1 nsec or less). FIGS. 16 and 18 show an example of the configuration of essential parts of a conventional laser diode driving circuit. The conventional laser diode driving circuit shown in FIG. 16 is arranged such that a current source 100 is connected to a laser diode 102 through a switching element 101, and this element 101 is driven to supply the laser diode 102 with a desired pulse current.
Further, another conventional laser diode driving circuit shown in FIG. 18 is designed such that a first current source 200 is directly connected to an anode of a laser diode 201, and a series circuit consisting of a second current source 203 for current sinking and a pseudo load 204 is connected to a junction or node between the first current source 200 and the laser diode 201 through a switching element 202, and this element 202 is driven to supply the laser diode 202 with a pulse current. The second current source 203 supplies an output current whose value is different from that supplied from the first current source 200.
The conventional laser diode driving circuits shown in FIGS. 16 and 18 will be described specifically. The laser diode driving circuit shown in FIG. 16 has a main part comprised of the current source 100 that supplies the laser diode 102 with a 100 mA current, and an NMOS transistor 101 functioning as the switching element connected between an anode of the laser diode 102 and the current source 100.
In the above configuration, as shown in FIG. 17A, for example, a 100 MHz pulse voltage signal is applied to a gate of the NMOS transistor 101 to switch the transistor 101. As a result, a current pulse whose height is 100 mA is generated at a node OUT1 and flows into the laser diode (FIG. 17B).
However, the potential at a node N1, which is a junction between the current source 100 and the NMOS transistor 101, increases to a level near a source voltage Vdd when the NMOS transistor 101 turns off, and hence this potential does not drop from such level near the source voltage Vdd immediately when the NMOS transistor 101 turns on again (FIG. 17C). As a result, the current pulse generated at the node OUT1 does not respond quickly to the pulse voltage signal (FIG. 17A) applied to the gate of the NMOS transistor 101 as shown in FIG. 17B, and thus the waveform of the current pulse is blunted.
Further, the conventional laser diode driving circuit shown in FIG. 18 has a main part comprised of the first current source 200 that supplies a 100 mA current to the laser diode 201, the NMOS transistor 202 as the switching element whose drain is connected to a junction between the first current source 200 and the laser diode 201, the second current source 203 for current sinking connected to a source of the NMOS transistor 202, for supplying an output current (50 mA) different from that supplied from the first current source 200, and the pseudo load 204 connected to the second current source 203.
In the above configuration, as shown in FIG. 19A, for example, a 100 MHz pulse voltage signal is applied to a gate of the NMOS transistor 202 to switch the transistor 202. As a result, during an on period of the NMOS transistor 202, a 50 mA current I2 flows from the first current source 200 into the current sinking second current source 203, and further flows to the pseudo load 204. Simultaneously, a 50 mA current I1 flows from the first current source 200 to the laser diode 201. On the other hand, during an off period of the NMOS transistor 202, the current I2 flowing into the second current source 203 assumes 0 mA, whereby the current I1 flowing from the first current source 200 to the laser diode 201 assumes 100 mA (FIGS. 19B and 19C).
When the NMOS transistor 202 is switched from on to off under this condition, the potential at a node OUT2 assumes the ground potential (0 V), and hence the 50 mA current does not flow to the pseudo load 204 immediately even when the NMOS transistor 202 turns on again. As a result, the current I1 flowing into the laser diode 201 cannot respond quickly to the pulse voltage signal (FIG. 19A) applied to the gate of the NMOS transistor 202, and thus the waveform of the current pulse is blunted.
It is therefore an object of the present invention to provide a laser diode driving circuit which is capable of supplying a laser diode thereof with a high-speed pulse current.
To attain the above object, in a first aspect of the present invention, there is provided a circuit for driving a laser diode, comprising a current source that supplies a current to the laser diode, a first switch connected between the current source and the laser diode, a second switch connected between a junction between the current source and the first switch and a pseudo load, and a controller that supplies a first voltage pulse signal to the first switch and a second voltage pulse signal opposite in phase to the first voltage pulse signal to the second switch to switch the first and second switches in a complementary manner.
According to the above configuration, a constant current always flows through the junction between the current source and the first switch so that the potential at the junction is kept from varying. As a result, the laser diode can be supplied with a high-speed pulse current.
Further, to attain the above object, in a second aspect of the present invention, there is provided a circuit for driving a laser diode having a cathode and an anode, comprising a first current source connected to the anode of the laser diode, the cathode of which is grounded, for supplying an offset current to the laser diode, a second current source that supplies a current for superimposition upon the offset current, a first switch connected between the second current source and the anode of the laser diode, a second switch connected between a junction between the second current source and the first switch and a pseudo load, and a controller that supplies a first voltage pulse signal to the first switch and a second voltage pulse signal opposite in phase to the first voltage pulse signal to the second switch to switch the first and second switches in a complementary manner.
According to the above configuration, a constant current always flows through the junction between the second current source and the first switch so that the potential at the junction is kept from varying. Therefore, the laser diode can be supplied with a high-speed pulse current wherein the current pulse obtained from the current supplied from the second current source by switching the first switch is superimposed upon the offset current from the first current source.
Further, to attain the above object, in a third aspect of the present invention, there is provided a circuit for driving a laser diode having a cathode and an anode, comprising a first current source connected between the anode of the laser diode, the cathode of which is grounded, for supplying an offset current to the laser diode, at least two second current sources each having an output, for supplying currents for superimposition upon the offset current, first switches connected between the outputs of the at least two second current sources and the anode of the laser diode, second switches connected between respective junctions between the outputs of the at least two current sources and the first switches and a pseudo load, and a controller that controls driving of the first and second switches such that the laser diode is supplied with a current obtained by superimposing a pulse current signal upon at least the offset current as a reference level, and such that amplitude of the pulse current signal is variable.
According to the above configuration, a constant current always flows through the junctions (nodes) between the respective outputs of the two or more second current sources and the first and second switches, whereby the potential at the nodes can be kept from varying. Therefore, the current supplied to the laser diode can be controlled such that the pulse current signal is superimposed upon at least the offset current as a reference level and such that the amplitude of the pulse current signal is variable.
Preferably, in the laser diode driving circuit according to the third aspect, the controller controls driving of the first and second switches such that the laser diode is supplied with a current obtained by first superimposing a second offset current upon a level of the offset current to obtain an offset current as a second reference level which is increased by the superimposition, and then superimposing a pulse current signal upon the offset current as the second reference level.
According to this configuration, in addition to the above-mentioned effects obtained by the third aspect, the current supplied to the laser diode can be controlled such that the offset current is variable and the pulse voltage signal is superimposed upon this offset current.
Further, to attain the above object, in a fourth aspect of the present invention, there is provided a circuit for driving a laser diode having a cathode and an anode, comprising a first current source having an output thereof connected to the anode of the laser diode, the cathode of which is grounded, for supplying an offset current to the laser diode, a second current source that supplies an output current whose value is different from that of an output current supplied from the first current source, a current mirror circuit having a first transistor for sinking the output current from the second current source, and a second transistor driven by the first transistor, a switch connected between the second transistor and a junction between the output of the first current source and the anode of the laser diode, and a controller that supplies a voltage pulse signal to the switch.
According to the configuration of the fourth aspect, a constant current always flows through the junction between the output of the first current source and the anode of the laser diode, whereby the potential at the node can be kept from varying. Therefore, by causing the switch to intermittently shunt a predetermined amount of the output current from the first current source to the current mirror circuit using the offset current from the first current source as a reference, a high-speed pulse current is obtained, and hence the laser diode can be supplied with such a high-speed pulse current.
Further, to attain the above object, a fifth aspect of the present invention provides a circuit for driving a laser diode having a cathode and an anode, comprising a first current source connected to the anode of the laser diode, the cathode of which is grounded, for supplying an offset current to the laser diode, a second current source having an output, for supplying an output current whose value is different from that of an output current supplied from the first current source, a current mirror circuit having a first transistor for sinking the output current from the second current source, and a second transistor driven by the first transistor, a first switch connected between the output of the second current source and the first transistor, a second switch connected between the output of the second current source and the second transistor, the first current source having an output thereof connected to a junction between the second switch and the second transistor, and a controller that controls driving of the first and second switches so as to switch in a complementary manner.
According to the above configuration, the laser diode can be supplied with a high-speed pulse current, and the amplitude of the current supplied to the laser diode can be varied with respect to a central value thereof which is fixed.
Preferably, in the laser diode driving circuit according to the fifth aspect, the circuit for driving a laser diode further comprises a third switch connected between the output of the second current source and the first and second switches, and a fourth switch connected to the first and second transistors constituting the current mirror circuit, and wherein the controller drives the third switch to block current supply to the first and second switches, and drives the fourth switch to forcibly turn off the first and second transistors, to limit a current supplied to the laser diode only to the offset current from the first current source.
According to this configuration, in the laser diode driving circuit according to the fifth aspect, in addition to the above-mentioned effects obtained by the fifth aspect, the influence of the operation performed by the current mirror circuit can be blocked quickly when the current supplied to the laser diode is limited only to the offset current from the first current source.
Further preferably, in the laser diode driving circuit according to the fifth aspect, the circuit for driving a laser diode further comprises a fourth switch connected to the first and second transistors constituting the current mirror circuit, and a fifth switch connected between the output of the second current source and a pseudo load, and wherein the controller drives the fourth switch to forcibly turn off the first and second transistors, and turn on the fifth switch to cause the output current from the second current source to flow to the pseudo load, to limit a current supplied to the laser diode only to the offset current from the first current source.
According to this configuration, a constant current always flows through the junction between the second current source and the first and second switches, and hence the potential at this junction can be kept from varying. Thus,a high response can be implemented in switching from the operating state of limiting the current supplied to the laser diode only to the offset current from the first current source to the operating state of generating a pulse current either by superimposing the current from the second current source upon the offset current through the operations of the first and second switches and the current mirror circuit, or by shunting a portion of the offset current which is equal in value to the current from the second current source, to the current mirror circuit.
Further, preferably, in the laser diode driving circuit according to any of the first to fifth aspects, the first and second transistors and/or the first and second switches are formed of MOS transistors.
According to this configuration, the laser diode driving circuit according to any of the first to fifth aspects can be formed on a CMOS semiconductor integrated circuit.
The above and other objects of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.